IN BRIEF: Something that kills the chances of a good outcome. This is mainly said when competition is involved.

IN BASKETBALL:
-A completed buzzer-beater at the end of the game to take the lead,
-a field goal or free throw made for a team with just enough time for the opposite team to not produce the win or
-if the lead is completely out of reach, "2:00 left" is automatically the dagger.

IN FOOTBALL:
-A completed touchdown, field-goal or a safety at the end of the game to take the lead,
-a touchdown, field goal or safety made for a team with just enough time for the opposite team to not produce the win or
-if the lead is completely out of reach, "2:00 left" is automatically the dagger.

EXAMPLE: Mo Williams shot the dagger in Cleveland's win against Atlanta in Game 4. Just so you know, Cleveland swept Atlanta in that game, too.